The present invention relates to a plasma deposition device for forming a thin film, especially a plasma deposition device for forming a film functioning as a semiconductor. More specifically, the present invention relates to a plasma deposition device for forming a thin film preferably utilizing a plasma-excited chemical vapor deposition method utilized for manufacturing an insulation film or a semiconductor film such as amorphous silicon (hereinafter referred to as a-Si) utilized in the electronic industry.
The method for manufacturing an electronic device such as an integrated circuit, a liquid crystal display, an amorphous solar battery and the like by depositing a semiconductor film and the like using plasma is called a plasma-excited chemical vapor deposition (CVD) method, which is advantageous in its simplicity and its maneuverability and is applied to manufacture various electronic devices.
The general CVD method will now be explained, with reference to FIGS. 8 and 9 showing the structure of a plasma deposition device (plasma CVD device) utilizing this general CVD method. FIG. 8 is a cross-sectional view explaining the concept of the structure of the plasma CVD device, and FIG. 9 is a perspective view showing the structure of the main portion of the device.
A prior-art plasma CVD device comprises a first electrode 13-1 mounted on the first surface of an electrode substrate 11, a gas supply space 15 formed to the back side of the electrode substrate 11, a deposition substrate 30 arranged to oppose to the first electrode 13-1 with a predetermined distance d in between, a second electrode 13-2 mounted to the back surface of the deposition substrate 30, a vacuum container 50, an induction terminal 51, a deposition substrate holder 52, a power source 60, and a gas supply unit 70. A plurality of gas introducing holes 12 are provided to the electrode substrate 11 and the first electrode 13-1 mounted thereto, supplying material gas G to plasma generation space 10. High-frequency output from the power source provides electric energy to the first electrode 13-1 and the second electrode 13-2. The gas supply unit 70 is connected via a gas supply tube 16 to the gas supply space 15, through which material gas for forming the thin film is supplied during deposition.
The plasma CVD device generates plasma by causing discharge DC to be performed between the first electrode 13-1 and the second electrode 13-2, which are two conducting plates mutually insulated and opposed to each other in parallel, and provides material gas G thereto so as to dissociate the gas and to generate radicals R. Thereby, a semiconductor film and the like is deposited on the deposition substrate 30 made of silicon or glass and mounted to the second electrode 13-2.
The means for generating plasma that resolves the material gas to be deposited utilizes a high-frequency power generally having a frequency of 13.56 MHz. That is, one conductor plate electrode 13-2 is set to ground potential, and high-frequency voltage is applied between the electrode 13-1 opposed thereto, thereby generating a high-frequency electric field between both conductor plates. This state of breakdown generates plasma as a glow discharge phenomenon. The electrode 13-1 to which high-frequency voltage is impressed is called the cathode electrode, and a large electric field is formed near the electrode, which accelerates the electron in the plasma and encourages dissociation of material gas, thereby generating radicals R.
Accompanied by the recent advancement in plasma engineering and semiconductor engineering, a new proposal has been made to the plasma CVD method. One example involves improving the deposition speed of the semiconductor film by increasing the frequency of the utilized high frequency output from 13.56 MHz to a VHF band (J. Vac. Sci. Technol. A10 (1992) 1080, A. A. Howling).
Electronic devices such as the liquid crystal display or the amorphous solar battery are especially large-sized electronic devices, and there is strong demand for a larger product formed by utilizing a deposition substrate 30 having a size ranging from the order of 10 cm square to 1 m square.
However, there is a limit to the prior art method related to forming a thin film by deposition to a deposition substrate 30 having a small size. A large-sized electronic device such as a liquid crystal display or an amorphous solar battery was difficult to manufacture according to the prior art method, since it was difficult to deposit a high-quality film having a uniform film thickness to a deposition substrate 30 having a large area.
One reason causing difficulty in securing a uniform film thickness is that when high frequency is used, the inductance of the material constituting electrodes 13-1 and 13-2 or the partial difference in electrical connection of the parts constituting the electrodes 13-1 and 13-2 causes high-frequency power that generates uneven plasma on the deposition substrate 30, resulting in uneven density distribution of the plasma particles and radical particles. As a result, the thickness of the film formed on the deposition substrate 30 varies locally.
In the case of a TFT (thin film transistor) liquid crystal display utilizing an a-Si film, if the thickness of the a-Si film functioning as the switching layer varies within one deposition substrate 30, the switching property is partially varied, and thus, the display becomes uneven. There is a demand for a method that reduces the uneven distribution of the plasma density, and enables to grow a film having a uniform thickness on the deposition substrate 30.
One reason causing difficulty in obtaining a high-quality deposition is that the deposition substrate 30 is mounted on ground electrode during deposition. When plasma is generated, a potential difference called a sheath voltage occurs on the surface of the deposition substrate 30 positioned above the ground electrode, and basically such potential difference cannot be avoided as long as plasma exists. Sheath voltage accelerates the ion within the plasma towards the deposition substrate, which results in ions providing impact to the surface of the deposited film, deteriorating the quality of the film.
A method is proposed in Japanese Patent Laid-Open Publication No. 11-144892 that improves the film-thickness distribution to the deposition substrate 30 and deposits a high-quality film. The disclosed method for manufacturing the film includes providing a plurality of electrodes having a wavy uneven surface, and providing the deposition substrate 30 away from the electrodes so as to form a horizontal electric field, thereby enabling to manufacture a uniform and high-quality film having a large size. However, according to this deposition method, if discharge electrodes are formed to have a width of a couple of millimeters, the cross-section of the electrodes can be shaped as a triangle, a trapezoid, a semicircle, or a T-shape and the like, which causes the height of the electrodes to be varied for a couple of millimeters. Thereby, the surfaces of the electrodes are not positioned at fixed distances from the deposition substrate. If a uniform deposition is to be formed under such condition, the deposition substrate 30 must be separated by a considerably long distance away from the surfaces of the electrode surfaces so as to reduce the ratio of dispersion of the distance between each electrode for deposition. According further to this method, during formation of discharge electrodes, the step for forming a wavy form to the electrode formation surface having a large area ranging from the order of 10 cm square to over 1 m square requires high mechanical accuracy. Moreover, since the distance between electrodes is fixed according to the structure, the Paschen property for plasma generation (the value of plasma-discharge-starting voltagexc3x97inter-electrode distance relativity) limits the range of operating voltage. Even further, since voltage is simultaneously applied to plural electrode pairs, a power source 60 capable of outputting high electrical power is necessary.
With consideration to the above prior-art problems, the present invention aims at providing a plasma deposition device capable of forming a uniform and high-quality film deposition on a large-sized deposition substrate, and to increase the number of products (such as liquid crystal panel) to be taken from one deposition substrate, thereby contributing to the improvement of productivity.
The object of the present invention is to provide a plasma deposition device capable of realizing a high-quality film deposition, that enables to provide not only a high-quality a-Si film utilized for TFT liquid crystal display, but also a silicon dioxide film, a silicon nitride film, or a crystalline silicon film.
The plasma deposition device for forming a thin film according to the present invention comprises a function of introducing material gas to the interior, a function of generating a plasma state from the material gas by providing electric energy thereto, a function of resolving the material gas into active species, and a function of depositing the active species on a deposition substrate and forming a thin film, wherein the device is equipped with a plurality of electrodes positioned separately from the deposition substrate and each having an exposed surface parallel to the surface of the deposition substrate, and electric energy is supplied to the device by applying voltage between the electrodes.
Preferably, the plasma deposition device for forming a thin film has plurality of electrodes arranged in a striped form.
More preferably, the plasma deposition device for forming a thin film has the surface of the plurality of electrodes covered with a dielectric layer.
According to a further preferable example, the plasma deposition device for forming a thin film introduces material gas to the interior through a plurality of introducing holes provided between the plurality of electrodes.
Moreover, the plasma deposition device for forming a thin film applies voltage providing electric energy to the device either as a low frequency or as a high frequency.
Further, the plasma deposition device for forming a thin film applies voltage providing electric energy to the device in the state of a direct pulse.
Preferably, the plasma deposition device for forming a thin film applies the voltage providing electric energy to the device in a temporally staggered state according to position.